Intravascular catheters, such as that disclosed in U.S. Pat. No. 5,762,636, have commonly been used for repeated drug administration and/or blood sampling on animals such as mice, rats, rabbits, dogs, cats, sheep, and human and non-human primates. However, the use of intravascular catheters in such applications, particularly for repeated drug administration over a period of time, poses an unacceptably high risk of infection of the open wound and of catheter disengagement and/or damage caused by movement or actions of the animal.
In extended use applications, it is therefore preferable to use surgically implanted ports as a means of delivering drugs or collecting bodily fluids. Subcutaneous ports generally comprise a housing defining a chamber, a septum disposed with one surface adjacent the dermis and an opposing surface adjacent the chamber, and an output. The chamber communicates with a delivery and/or collection site in the patient or subject through the output, which often includes a catheter.
Once the port is implanted and the chamber filled with a drug, a bolus or therapeutic dose of the drug contained in the chamber is delivered at a desired rate to a particular location within a patient's or subject's body, such as a cavity, a large vein, a tumor, or an injury site. The septum comprises an elastomeric material that is penetrable by a needle and is self-sealing to mitigate the leakage that would otherwise result from repeated punctures from injections or fluid collection. The chamber may be replenished by inserting a hypodermic needle through the patient's or subject's skin and the septum and injecting the drug into the chamber.
Ports therefore permit a means for efficiently dispensing a drug or medicament into the body of a patient or subject over a prolonged period of time by means of a single injection (e.g., continuous infusion) or repeated intermittent injections. Likewise, ports offer the opportunity for collection of fluids (e.g., blood) from a designated collection site adjacent an outlet of a catheter attached to the port. For example, with respect to dogs and cats, a catheter attached to the port may used to sample blood from the cephalic, recurrent metatarsal, jugular, and femoral veins. However, sampling is feasible, but generally not advisable, for small needles (e.g., 22-gauge) due to the potential for plugging or occlusion of the needle. Sampling using needles of 20-gauge, 18-gauge, or large is generally acceptable. Despite the real advantages provided by implantable ports, conventional ports suffer from numerous disadvantages, some of which are described below with respect to two conventional ports.
U.S. Pat. No. 5,848,989 provides one example of an implantable port for delivering and collecting fluids in a body particularly for use with laboratory animals. This port has a hollow housing 2 in which a relatively small inlet 7 is defined by a radially inwardly projecting lip 18 which is substantially perpendicular to the base 17 of the housing. A septum 4 is mounted in the housing adjacent a mouth of the inlet and is then compressed against the housing inlet lip 18 by a hollow core 3 inserted within the housing in a compression fit. The housing 2 has an open flared end 5 extending in front of the inlet 7 and having a surface shaped to guide a needle to the port inlet and into the septum.
However, this configuration presents many disadvantages including a prismatic shape which both resists manual fixation and presents edges which can damage tissue if the edges are pressed against the surrounding tissue during manual fixation prior to insertion of a needle. Moreover, the septum is obscured by the inlet lip and, in combination with open flared end of the housing which serves to guide a needle to the inlet 7 during use, the risk of damage to the needle tip upon initial needle insertion is significant. Further, once the septum is penetrated, the required angle of insertion is likely to cause the needle tip to immediately impact against an interior wall of the hollow core 3 chamber 8. Each needle insertion therefore presents a hazard during both needle insertion and needle withdrawal, as the damaged needle tip cuts lesions or tears into the septum. Another hazard is presented as the needle outer diameter increases relative to the septum diameter.
Such tears and lesions provide an ideal environment for the growth of bacteria in the subcutaneous environment, increase the risk of infection and/or tissue necrosis, and increase leakage from the septum. These factors reduce the useful life of the port and may necessitate an additional surgical procedure to removal the damaged port and insert a new port, with the attendant risks of infection and trauma to the animal and possibly loss of test data. In practice, the interference fit between the housing 2 and the hollow core 3 renders disassembly and reassembly of the port to replace septum 4 during surgery highly impractical at the very least and the entire port replaced.
An additional disadvantage in the device disclosed in U.S. Pat. No. 5,848,989 arises from the asymmetric geometry and angles surfaces (e.g., corners) presented thereby. When any port is implanted in a body, fibroblasts lay down dense fibrous tissue (collagen, elastin, reticular) in reaction to the foreign body so as to isolate the foreign body. The material and shape of the foreign body influences the thickness of the fibrous capsule (encapsulation). The asymmetric shape of the aforementioned device with a relatively larger inlet end, the presence of the septum at the inlet end, the repeated punctures of the septum (and formation of lesions therein), and repeated punctures of the fibrous capsule all contribute to a disproportionate encapsulation or fibrosis (scar tissue formation) at the front or inlet end. Over time, this disproportionate encapsulation causes migration of the port away from the original site in a direction of the outlet, particularly when absorbable sutures are used. Migration of the port may cause the catheter to kink or bend causing occlusion and/or damage, particularly for small diameter catheters (e.g., French size 3 or 4, having an outer diameter of about 0.9 mm and 1.20 mm, respectively).
German Patent No. 0474266 provides another example of an implantable catheter device for a small experimental animal characterized a tubular housing 10 having a cavity 13 closed at an end face thereof by a penetrable septum 14. The other end area of the housing 10 forms a connection part 30 in which a clamping space 21 receives an elastomeric clamping piece 11 surrounding a catheter 12. A portion of the connection part 30 projecting beyond clamping piece 11 has internal threads 31 matingly engageable with an external thread of pressure piece 20 shaft 32. The end face of the externally threaded shaft 32 axially compresses clamping piece 11 to press it against catheter 12 provided on a rigid supporting cannula 22. The cannula's lumen communicates with the cavity 13, on one side, and the pressure piece 20 and associated catheter 12 on the other side.
However, similar to the device of U.S. Pat. No. 5,848,989, the retaining ring 15 flange 16 of this port defines a small septum target area, relative to the outside diameter of the port housing, that is inwardly displaced from the outer edge of the flange. This configuration presents an increased risk for needle damage and associated complications, as noted above. Further, the minimal cavity 13 depth, combined with the required angle of needle insertion, provides an almost imperceptible clearance for a needle the tip between the septum inner wall and the cavity 13 wall, increases the probability of damage to the needle tip upon insertion and damage to the septum upon withdrawal of the needle. Additionally, if the integrity of the septum is compromised, the connection part 30 and the retaining ring 15 welded thereto must be removed and discarded.
Thus, a need exists for an implantable subcutaneous port which avoids the above-noted deficiencies of conventional subcutaneous ports.